Question { Reddy Labs, 25843 }

How can we calibrate pressure and flow transmitter in the
field(online)?

Answer

This procedure can be used to calibrate any type of
instrument in the field on a live plant regardless of it's
type or application. The only variation in the procedure is
what test equipment you will use to calibrate the instrument
with. For instance calibrating a temperature transmitter you
want to take a HART and decade box with you. Calibrating a
pressure , flow or DP transmitter, you want to take a HART
and pressure pump with you like a Druck or hand pump with a
precision gauge on.
The procedure:
Since the transmitter is on line and on a running plant, you
need to determine a couple of things first before you start
working on it. First and most important of all, what does it
do? Is it a control transmitter, a trip transmitter or just
a indication.
Control transmitter means it's output will go to the DCS and
is linked to a PID controller which will give a continues
output, based on this input, to a control element in the
plant plant like a control valve or a heater element. These
are all critical control components that will trip the plant
if you upset their stability.
Trip transmitter means that it is linked to the ESD system
and will start a plant shut down if it is seeing a to high
or low pressure, flow, DP or level depending on the
application you are working on. Just indication means that
it is completely safe to work on and you cannot trip anything.
To make the control transmitter safe to work on you need to
put the DCS PID controller in manual. This will keep the
output of the controller at the current output position and
bypass the output function of the controller. In a fast
moving application the control room operator will send a
field operator with you to control the control valve
manually with it's hand wheel while you work on the control
transmitter.
To make the trip transmitter safe to work on you need to put
a inhibit in the ESD system output trip function of the
transmitter. This is also sometimes refer to as a MOS
(maintenance override switch).
In both cases the control room operators will do this for
you providing that you have all the permit to work and
inhibit permission paperwork in place.
Good luck

Question { HPCL, 34442 }

What is the difference between compensation and extension cable.

Answer

No difference it is just a different wording for the same
thing. Extension or compensation wire is made from the same
material as the used thermocouple to extend the thermocouple
to the Temperature indicator or controller. So you therefore
get Type K, J, T S ext types of extension wire as well as
Type J, K , T ext types of connectors to join these cables
as well.

Question { 9480 }

is here anybody who knows how to calibrate the pressure
transmitter? ONLINE

Answer

This procedure can be used to calibrate any type of
instrument in the field on a live plant regardless of it's
type or application. The only variation in the procedure is
what test equipment you will use to calibrate the instrument
with. For instance calibrating a temperature transmitter you
want to take a HART and decade box with you. Calibrating a
pressure , flow or DP transmitter, you want to take a HART
and pressure pump with you like a Druck or hand pump with a
precision gauge on.
The procedure:
Since the transmitter is on line and on a running plant, you
need to determine a couple of things first before you start
working on it. First and most important of all, what does it
do? Is it a control transmitter, a trip transmitter or just
a indication.
Control transmitter means it's output will go to the DCS and
is linked to a PID controller which will give a continues
output, based on this input, to a control element in the
plant plant like a control valve or a heater element. These
are all critical control components that will trip the plant
if you upset their stability.
Trip transmitter means that it is linked to the ESD system
and will start a plant shut down if it is seeing a to high
or low pressure, flow, DP or level depending on the
application you are working on. Just indication means that
it is completely safe to work on and you cannot trip anything.
To make the control transmitter safe to work on you need to
put the DCS PID controller in manual. This will keep the
output of the controller at the current output position and
bypass the output function of the controller. In a fast
moving application the control room operator will send a
field operator with you to control the control valve
manually with it's hand wheel while you work on the control
transmitter.
To make the trip transmitter safe to work on you need to put
a inhibit in the ESD system output trip function of the
transmitter. This is also sometimes refer to as a MOS
(maintenance override switch).
In both cases the control room operators will do this for
you providing that you have all the permit to work and
inhibit permission paperwork in place.
Good luck

Question { Alstom, 9890 }

what is burner management system?

Answer

Use in steam boilers as the auto control sequence (DCS or
PLC controlled) for the various gas or oil burners.

Question { Chevron Phillips, 41607 }

how can we calibrate RADAR LEVEL transmitter 0% to 100%

Answer

You need to be more specific in your questions but I assume
you are referring to the frequency modulation radar (cone
type) and not the wave guide pulse radar (TDR).

This is a very generic procedure, use your radar's vendor
manual to do the calibration.

Take a measuring tape and measure the actual distance from
the cone to the zero position and the 100% position in your
vessel. If it is a closed pressurized vessel use the design
engineer's internal vessel drawings. If all else fail use
the dimensions of the vessel itself to determine what and
where z/100% is on the vessel. Use the vendor manual to see
how to use these measurements. It might be as simple as
entering these values directly as your z/100% calibration
values or you might have to do a small calculation like
maximum possible distance (tank depth) minus z measured
distance equals zero distance, and maximum possible (tank
depth) distance minus 100% measured distance equals 100%
distance.

Also keep in mind radars are distance measuring instruments
and not level measuring instruments and the fact that the
distance measurement is the exact opposite of level in
relation to the total distance measurement. This means if
your tank total distance is 3 meter (assuming tank bottom is
also zero position) and your radar measures a DISTANCE of 2
meters to the top of your product in the tank, the actual
LEVEL in the tank is only 1 meter.
It is easy to look at the disply on the radar and think this
is your actual level measurment. You therefore need to
modify the local display to indicate level in mm or %, as
well as setup your output to level and not distance after
the calibration. The radar will then give out 4mA when the
tank is empty as per example above and 100% as the tank fill
up with product to the point where you have specified 100%
should be. If your output is setup for distance it will
output the exact opposite. Watch out!!

If you need a vendor manual download it from the internet.
Good luck

Question { Chevron Phillips, 4600 }

ON RADAR INTERFACE LEVEL TRANSMITTER IF DI-ELECRIC IS WRONG
WITH CONTINIUS PROCESS THAN HOW CAN INDENTIFY OR HOW CAN I
PUT NEW DI-ELECTRIC VALUE?(ROSEMOUNT-TDR-3300-COXIAL PROBE
TYPE)

Answer

You said interface level so I must assume you are measuring
oil and water. My experience is on the Khrone BM100 A but
your radar might be similar since the both use TDR technology.
The dielectric constant of crude is about 2 to 4 and water
is 80. The variance in dielectric constant will have a small
effect, so in order to find the right valve one quick way is
to use your sight glass to set the dielectric constants so
that the radar reads the same as the sight glass on the
water and oil. a Good average is normally 2,5 and 80.
Unfortunately this will not work unless you have setup your
Z/S parameters correctly.
If you are still having problems you need to do a complete
setup from scratch. In order to do this you need to get hold
of the design engineer's internal vessel drawings and look
what the calibrated span should be. What you are interested
in is the exact mm measurements from vessel bottom to Z/S
points or positions. From there it is just a matter of
taking exact measurements in the field of your vessel and
your radar installation and make yourself a neat, accurate
detailed drawing to indicate the design spec positions and
the actual position of your probe in relation to the vessel,
and put these measured values in the output 1 and output 2
parameters, keeping in mind zero position is measured from
the probe bottom up and 100% is also from the probe bottom
up and not from zero position up. So if you can see where
the design engineer have said zero should be and you can see
in exactly what position your probe is in relation to those
points it is a matter of calculating how high you need to
measure up from probe bottom to get to those Z/S points
marked by the design engineer. It takes a bit of
trigonometry to do but is easy enough.

Typically these parameters should look something similar to
this. Output 1 (Top product level) 4mA = 150mm, 20mA =
2500mm, Output 2 (Bottom product) 4mA = 150mm, 20mA = 2500mm.
Look strange I know but we have found it is better to set
them both the same instead of trying to set each one to it's
individual span. Both Spans are setup in reference to actual
vessel level %. Interface is also a actual level measurment
in relation to the whole vessel and not to just half the
vessel where for instance to where your wier plate is. If
you do this it will have the effect that you have a very
sensitive interface measurement to a slow level measurment
making your control very difficult. We have done it at one
point and it worked eventually, but it was very difficult to
optimize and get stable control, and even the smallest upset
will cause everything to go crazy again.

In this example the 150mm might be the mm you need to
measure from the probe bottom up to get to the Zero position
of the actual vessel as indicated by the design drawing and
the same with the 2500mm. Obviously just examples. This is
where your exact measurements in the field comes in.
NB!!
Also make sure you have the right probe length in the "tank
Height" parameter and not the real vessel height.
This probe length is normally stamped on the little spec
plate on the head by the supplier.
Good luck!

Question { 8702 }

As an industrial instrument technician what we have to know?

Answer

I think you should wait until you can answer this question
yourself before calling yourself a industrial instrument
technician.

Question { Aker Solutions, 9429 }

why we use double acting valve and where?

Answer

Normally a control valve is refer to by it's fail position.
This means "what position will the valve move to should the
supply air or control signal to the valve falls away". This
is important to safe guard the process at various places so
some valves will be fail open and some fail close. In order
to have valve as a fail open or close the valve the actuator
have to be spring loaded. So by having the spring on top or
bottom of the actuator piston, will determine if it will be
a FO or FC valve. This kind of valve is also called single
action since it will only have one output from its
positioner to either the top or bottom of the actuator. The
positioner on the valve is also setup as a single acting
positioner since it will only give a single action to the
actuator, the reverse action will be done by the spring. The
problem with this setup is that it is possible that the
process might be so strong or the pressure so high (during a
blow down or ESD shutdown in the plant) that the spring
might in certain instances be to week to push the valve into
the fail position quick enough, due to the back pressure
from the process and can cause damage to the plant or even a
explosion. To make sure that the valve will go to the fail
position we install a double action positioner with two
outputs. One goes to the top of the actuator and one to the
bottom. This is also very helpful to do very accurate and
stable control on a high flow line since the pressure from
the position do the actual control and not spring control
one way and positioner control the other way as in single
acting control valves. It is also solving the problem that
the valve will now be forced into the fail position by the
spring as well as the positioner supply pressure during a
emergency.
In shutdown valves (open/close ESDV's) the same is true and
sometime at critical and high pressure points we use
hydraulics instead of pneumatics as the double acting agent
to make sure the valve will close during a emergency.
So to summarize the double acting action in ESD and control
valve is just there to make sure the valve will do what it
was designed for. Call it a extra fail safe if you want. In
theory not needed since a single acting valve should do the
trick just as well,but in practice you are at time very glad
you did it especially if you look at the kind of pressures
the valves are working on. With those kind of flows and
pressures you don't want to leave anything to chance.

Question { 4045 }

How can you differentiate Instrumentation to Electrical?

Answer

Instrumentation is all about measurement and control of
industrial processes like flow, pressure, level, density,
viscosity and so on. This is done with various sensors in
the plant that are calibrated by instrument technicians
according to the relevant process or application. The
readings of these sensors are then send to a central control
room via a world standard 4 to 20 mA signal and 24VDC, where
a monitoring supervisory system displays the information on
a computer screen. Control room operators interact with
these computer screens and stop and start pumps in the field
by clicking with the mouse on a small drawing on the screen
of the relevant pump. How this is done is with SCADA or DCS
software normally programmed by the instrument technician as
well. Instrumentation consists of only two sections, the
field side and the software side. Most instrumentation
technicians can do both.
Electrical work is done by a electrician and is all about
main power supply and distribution to the complete plant
either by using power from the main grit or local power
generators on the plant. Normally instrument technicians
will work on up to maximum 110VAC, a electrician can work
on any kind of power low or high. Only qualified high
voltage electricians are allowed to do high voltage
switching since this can be very dangerous to to.

Question { 36817 }

what is dry leg and wet leg in transmeter calibration?

Answer

We use these configurations during the calibration of a
Differential Pressure Transmitter (in short DP Cell) in a
level calibration of a closed pressurized vessel. This can
only be used when you are making use of a Diff Press
Transmitter that is piped to the high and low tap off points
on the vessel with stainless steel piping. You cannot use it
on any other type of level measurement device, even if it is
also a Diff Press Transmitter with capillary tubes and pad
cells installed on the H/L tap off points and not stainless
steel piping. When you use capillaries you need to do the
calibration completely differently from normal, so be
careful when using capillaries in level applications.
Ok back to wet and dry leg calibrations.
The dry leg is the most common and the easiest to do. This
is much the same as the basic open tank level calibration.
The transmitter is mounted anywhere below the HP (bottom)
tap off point and it's HP leg is connected via S/S tubing to
the HP (Bottom) tap off point on the vessel. The LP side of
the transmitter is connected to the LP (Top) tap off point
on the vessel. The HP side will always be in contact with
the liquid in the vessel and the LP side will always be in
contact with gas since it's is tapped of from the top of the
vessel. You obviously can only achieve this if you have a
5-way manifold (isolation, vent and equalization valve
piece)installed on the transmitter.
You will start your calibration by opening up the
transmitter to atmosphere and make sure that when equal
press is applied to HP and LP side the transmitter shows
zero and 4 mA. After this zero check it is a simple matter
of measuring where your Zero and 100% positions are on the
vessel in relation to the transmitter and multiply these
with the density of the liquid you are measuring and and
install these Z AND 100% values in the transmitter.
Ok this is very easy so far but what happens when the liquid
is hotter than the ambient temperature and it's vapor in the
top half of the vessel starts to condense and run into the
dry LP leg?
In a very short time this dry leg is going to start filling
up with condensate and there goes your calibration because
the calibrated diff press (your calculated Zero and 100%
values) begins to chance.
To resolve this problem we fill the LP leg with a buffer
solution like diesel,glycerin, glycol or even the same
liquid you have in your vessel can work as well, in non
critical applications. I prefer glycol since it's density is
higher than water so if the gas starts to condensate it will
just lie on top of the glycol buffer solution and run back
into the vessel from the LP leg and not mix with it. The
mixing of the wet leg liquid with the gas condensate could
also cause problems and inaccuracies, since this could
chance the buffer density over a period of time.
To calibrate the transmitter will depend on the type and era
of transmitter you are using. The following calibration is
for smart transmitters only.
The smart transmitters that we use today can measure in the
negative (-1Bar) and you can do your calibration as normal.
The final result will be something like this, LRV =
-1230mmH2o (4mA), URV = +125mmH2o (20mA). I know it looks a
bit strange when you see it for the first time but here is
how it works.

Before you can do this calibration you need to know the ATM
value for the installation. The atmospheric value (ATM) can
be read directly from the transmitter by disconnecting the
HP side(Bottom) and open it up to atmosphere, so the only
pressure on the transmitter is on the LP side and this will
obviously push the transmitter into the negative.
Maximum negative differential pressure for a instalation =
ATM pressure.
Make sure the LP line is filled to the position where it
will start to run back into the vessel, then read off the
displayed value on the transmitter. This is your ATM value.
In this example it might be something like -1350mmH2o. This
value is determined by, where you have installed the
transmitter and what you use for a buffer solution.
To calculate the actual zero and 100% positions on the
vessel you do the same as before and just measure from the
transmitter to you zero and 100% positions on the vessel,
multiply them with the density of the liquid you are
measuring and add them to the ATM value. You can then input
these values to this transmitter's LRV and URV and the
calibration is done.
So assuming you have installed the transmitter slightly
below the lower tap off point the above LRV and URV is about
right in relation to the ATM value in this example. Be sure
to understand the difference between the ATM value and the
LRV it will in most cases not be the same. The more
accurately you can determine your ATM value the more
accurate the calibration will be.
Now the calibration of the 4to20mA and the pneumatic DP
transmitters. These transmitters cannot measure in the
negative so you need to change the HP and LP sides around so
that the HP side goes to the top of the vessel and the LP
side goes to the bottom tap off point on the vessel.
You now need to do you calibration in the reverse as well.
Again find the ATM value first, in other words max positive
differential (HP wet leg filled and LP open to atmosphere)
on the transmitter will now be your ATM value. Will be say
+1350mmH2o.
Actual zero will now be 20mA and not 4mA and will be
determined by makind use of the ATM value minus the actual
zero measured value, multiplied by the liquid density.
The actual 100% value will be determined by making use of
the ATM value minus the actual 100% measured value,
multiplied by the density. You should end up with something
like this, zero = +1250mmH3o = 20mA and 100% = +150mmH20 = 4mA.
Finally the display on you remote level indicator needs to
be changed as well otherwise it will read in the reverse. If
you use a pneumatic DP Transmitter just substitute 4 and 20
mA with 20 to 100Kps or 3 to 15 Psi the principle stays the
same.
There you have it, wet and dry leg calibrations used ONLY in
PIPED DP Cell level calibrations.
Good luck

Question { 9935 }

what is instrumentation?

Answer

The shortest answer would be measurement, control and
monitoring of various control processes and machinery.

Question { 7804 }

which is the best type to measure level of a Tank(water).
Displacer or Differential Pressure method?

Answer

If you have the money buy a DP Transmitter, they are much
easier to work with and more accurate, Dis placers are old
technology get rid of it. If you are measuring clean water
you can use stainless steel tubing and a 5-way manifold.

Here are some more info to do the calibration.
We use these configurations during the calibration of a
Differential Pressure Transmitter (in short DP Cell) in a
level calibration of a closed pressurized vessel. This can
only be used when you are making use of a Diff Press
Transmitter that is piped to the high and low tap off points
on the vessel with stainless steel piping. You cannot use it
on any other type of level measurement device, even if it is
also a Diff Press Transmitter with capillary tubes and pad
cells installed on the H/L tap off points and not stainless
steel piping. When you use capillaries you need to do the
calibration completely differently from normal, so be
careful when using capillaries in level applications.
Ok back to wet and dry leg calibrations.
The dry leg is the most common and the easiest to do. This
is much the same as the basic open tank level calibration.
The transmitter is mounted anywhere below the HP (bottom)
tap off point and it's HP leg is connected via S/S tubing to
the HP (Bottom) tap off point on the vessel. The LP side of
the transmitter is connected to the LP (Top) tap off point
on the vessel. The HP side will always be in contact with
the liquid in the vessel and the LP side will always be in
contact with gas since it's is tapped of from the top of the
vessel. You obviously can only achieve this if you have a
5-way manifold (isolation, vent and equalization valve
piece)installed on the transmitter.
You will start your calibration by opening up the
transmitter to atmosphere and make sure that when equal
press is applied to HP and LP side the transmitter shows
zero and 4 mA. After this zero check it is a simple matter
of measuring where your Zero and 100% positions are on the
vessel in relation to the transmitter and multiply these
with the density of the liquid you are measuring and and
install these Z AND 100% values in the transmitter.
Ok this is very easy so far but what happens when the liquid
is hotter than the ambient temperature and it's vapor in the
top half of the vessel starts to condense and run into the
dry LP leg?
In a very short time this dry leg is going to start filling
up with condensate and there goes your calibration because
the calibrated diff press (your calculated Zero and 100%
values) begins to chance.
To resolve this problem we fill the LP leg with a buffer
solution like diesel,glycerin, glycol or even the same
liquid you have in your vessel can work as well, in non
critical applications. I prefer glycol since it's density is
higher than water so if the gas starts to condensate it will
just lie on top of the glycol buffer solution and run back
into the vessel from the LP leg and not mix with it. The
mixing of the wet leg liquid with the gas condensate could
also cause problems and inaccuracies, since this could
chance the buffer density over a period of time.
To calibrate the transmitter will depend on the type and era
of transmitter you are using. The following calibration is
for smart transmitters only.
The smart transmitters that we use today can measure in the
negative (-1Bar) and you can do your calibration as normal.
The final result will be something like this, LRV =
-1230mmH2o (4mA), URV = +125mmH2o (20mA). I know it looks a
bit strange when you see it for the first time but here is
how it works.

Before you can do this calibration you need to know the ATM
value for the installation. The atmospheric value (ATM) can
be read directly from the transmitter by disconnecting the
HP side(Bottom) and open it up to atmosphere, so the only
pressure on the transmitter is on the LP side and this will
obviously push the transmitter into the negative.
Maximum negative differential pressure for a instalation =
ATM pressure.
Make sure the LP line is filled to the position where it
will start to run back into the vessel, then read off the
displayed value on the transmitter. This is your ATM value.
In this example it might be something like -1350mmH2o. This
value is determined by, where you have installed the
transmitter and what you use for a buffer solution.
To calculate the actual zero and 100% positions on the
vessel you do the same as before and just measure from the
transmitter to you zero and 100% positions on the vessel,
multiply them with the density of the liquid you are
measuring and add them to the ATM value. You can then input
these values to this transmitter's LRV and URV and the
calibration is done.
So assuming you have installed the transmitter slightly
below the lower tap off point the above LRV and URV is about
right in relation to the ATM value in this example. Be sure
to understand the difference between the ATM value and the
LRV it will in most cases not be the same. The more
accurately you can determine your ATM value the more
accurate the calibration will be.
Now the calibration of the 4to20mA and the pneumatic DP
transmitters. These transmitters cannot measure in the
negative so you need to change the HP and LP sides around so
that the HP side goes to the top of the vessel and the LP
side goes to the bottom tap off point on the vessel.
You now need to do you calibration in the reverse as well.
Again find the ATM value first, in other words max positive
differential (HP wet leg filled and LP open to atmosphere)
on the transmitter will now be your ATM value. Will be say
+1350mmH2o.
Actual zero will now be 20mA and not 4mA and will be
determined by makind use of the ATM value minus the actual
zero measured value, multiplied by the liquid density.
The actual 100% value will be determined by making use of
the ATM value minus the actual 100% measured value,
multiplied by the density. You should end up with something
like this, zero = +1250mmH3o = 20mA and 100% = +150mmH20 = 4mA.
Finally the display on you remote level indicator needs to
be changed as well otherwise it will read in the reverse. If
you use a pneumatic DP Transmitter just substitute 4 and 20
mA with 20 to 100Kps or 3 to 15 Psi the principle stays the
same.
There you have it, wet and dry leg calibrations used ONLY in
PIPED DP Cell level calibrations.
Good luck

Question { 28195 }

Explain Split Range Control In Control Valve Briefly.

Answer

Split range is as per answer 2 and not answer 1. Answer 1 is
using two valves, one is a fail open and one is a fail
close. The 4 to 20mA control signal is then controlling the
two valves but in opposite directions. This can also be done
by calibrating the one positioner as a direct acting and the
other as a reverse acting.
Yes this is 100% correct and can be done and the two valves
will work perfectly and in exactly opposite directions to
each other. In a case like this the two valves will have to
be identical as well otherwise the balancing act you are
trying to perform with this setup might not work.
Never seen a application where you would need this but if it
needs to work like that, it can be done.

Split range means you are splitting a 4 to 20mA control
signal in two.

This is very useful in applications where the product flow
increases very rapidly and then falls away again. a Good
example will be before a slug catcher where the incoming
product from the well heads is coming in suddenly very fast
and then falls away again after a couple of minutes.
Another application is to control the steam from a boiler.
At times you need a lot of steam and other times you only
need a small amount.
How do you size the valve needed to control these very big
changes. a Small valve is needed to control the normal flow
but if the flow increases the small valve will open fully
and it will still be to small to control the now very high
flow or pressure. If you install a big valve to control the
big pressure or flow the valve will only open 2 to 5% most
of the time and it will be impossible to do stable control
with such a valve opening. Say nothing about the plug and
seat that will only last a couple of days.
To control these very high variance in flow and pressures of
the process we install two valves in parallel in the same
line. Normally the one is a small valve and the other is
about twice the size of the small valve.
The design and process engineers will decide what minimum,
normal and maximum conditions are and do the valve sizing
accordingly.
We calibrate the small valve to open from fully close to
fully open with a signal of 4 to 12 mA. We then calibrate
the second bigger valve to open from fully close to full
open with a signal of 12 to 20 mA.

In a situation where you use identical valves you can take
the signal from one controller and send it to both valves
but in the above application it is better to use two
controller but with the same input from one pressure or flow
transmitter. Each valve will then be controlled individually
from its own controller and on it's own PID tuning set. I am
sure you can see that it will be quite impossible to find a
PID tuning set that are appropriate for both the big and the
small valve and it is therefore better to use two
controllers but with the same input.

During normal operations the small valve will control the
process without any problems based on the input from the
pressure or flow transmitter, and the bigger valve stays
close all the time. If the process changes to something
bigger than what the small valve can handle the second
bigger valve needs to help, and will start opening up, once
the small valve is fully open. Once the demand falls away
the big valve will start to close and then the smaller valve
until the process is back to normal operating conditions..
Both valves receive a full 4 to 20mA signal from it's own
controller, but will only react based on the 4 to 12 or 12
to 20mA calibration that was done on each valve's positioner.

In other simpler applications you can use two identical
vales and set their positioners as above and send the one
controller output signal to both. One PID tuning set will
also work for both valves and conditions.

The valve sizes will depend on what the conditions are so
they can be the same or they might need to be different sizes.
Good luck

Question { 3910 }

What is the deffernce bet. Instrumentation and Electrical
Enginerring?

Answer

Instrumentation is all about measurement and control of
industrial processes like flow, pressure, level, density,
viscosity and so on. This is done with various sensors in
the plant that are calibrated by instrument technicians
according to the relevant process or application. The
readings of these sensors are then send to a central control
room via a world standard 4 to 20 mA signal and 24VDC, where
a monitoring supervisory system displays the information on
a computer screen. Control room operators interact with
these computer screens and stop and start pumps in the field
by clicking with the mouse on a small drawing on the screen
of the relevant pump. How this is done is with SCADA or DCS
software normally programmed by the instrument technician as
well. Instrumentation consists of only two sections, the
field side and the software side. Most instrumentation
technicians can do both.
Electrical work is done by a electrician and is all about
main power supply and distribution to the complete plant
either by using power from the main grit or local power
generators on the plant. Normally instrument technicians
will work on up to maximum 110VAC, a electrician can work
on any kind of power low or high. Only qualified high
voltage electricians are allowed to do high voltage
switching since this can be very dangerous to to.
Good luck

Question { 10364 }

How many types of Control Modes in process control, plz
tell me all.....?

Answer

There is only two modes of control if you really think about it.
On/Off Control and PID Control.

Where some confusion can come in, is when you look at he
various CONTROLLER modes available on a DCS.
PID controllers are also sometimes linked together like in
cascade or feed forward control configuration, but the
control modes is still PID control.

Look at On/Off control:
You can have a open/close control valve in the field that is
controlled by the ESD or DCS system that will either open or
close this valve.
The ESD system do not do any control but it can open or
close a valve during a emergency as a step in it's blow down
or shut down sequence. This is in general not consider as
control just a action.
You can also have another open/close control valve in the
field that is controlling the level in a tank. In this case
it is connected to a Decap controller in the DCS. a Dcap
controller have a set-point that can be set to control the
level of the tank, but if a on/off valve is used as is, this
valve will open and close all the time to try and keep the
level on the set-point. So to prevent this the controller
have a small dead band around the set-point that can be
adjusted. This will then prevent the valve from opening and
closing until it reaches the dead-band threshold position.
Normally about 5% over and under the set-point. It is still
on/off control.

Looking at PID control:
Any mode in conjunction with proportional control is
possible on a PID controller it just depends on the
application. For example PID, PI, PD, P.
You can see them each as individual control modes if you
want, but I prefer to just stick to PID or on/off control
modes since there is a definite and distinct control mode
difference between the two. I see Dcap, PI, PD and P modes
as sub divisions on the two main control modes.
P = Proportional
I = Integral
D = Derivative
Good Luck